\(H_0\)  tension: As noted in PCP13, the best-fit Planck value of
\(H_0\)  differs from direct measurements based on the Cepheid-Type Ia
supernova distance ladder [1]}, [2]}, [3]}. The latest value from the SH0ESSupernovae,
\(H_0\) , for the Equation of State of Dark Energy. collaboration
[4]} is \(H_0 = 74.03 \pm 1.42 ~\text{km}~\text{s}^{-1} \text{Mpc}^{-1}\) , differing by
\(6.59 ~\text{km}~\text{s}^{-1} \text{Mpc}^{-1}\)  from the 12.5HMcl TTTEEE value of \(H_0\)  listed in Table
REF . Interestingly, the statistical significance of the discrepancy
between Planck and SH0ES has grown from \(2.5 \sigma \)  in PCP13 to \(4.3\sigma \)  today. There
are also hints, for example, from strong gravitational lensing time delays [5]}
that the late time value of \(H_0\)  may differ from the Planck value. The \(H_0\)  discrepancy is
perhaps the most intriguing tension with the base \(\rm {\Lambda CDM}\)  model at this time. There is a general consensus, from
application of the inverse distance ladder to BAO and Type Ia supernovae data, that this discrepancy,
if real, requires physics that reduces the value of the sound horizon \(r_{\rm drag}\)  irrespective of the
nature of dark energy [6]}, [7]}, [8]}, [9]}, [10]}. For a review of possible theoretical explanations of this
tension see [11]}.
